1. Field of the Invention
The invention relates generally to digital-to-analog converters and analog-to-digital converters specifically with respect to a digital-to-analog converter providing bi-polar and uni-polar output formats utilizing a single converter.
2. Description of the Prior Art
Present day digital data handling systems of the type, for example, utilized for aircraft control and data management require analog-to-digital conversion at the input to the system for converting analog data signals to the digital format required by the system and digital-to-analog conversion at the output of the system to convert the digital numbers provided by the system into the analog signals required by utilization devices. The analog signals provided to such systems and the analog signals required by the utilization devices generally have both uni-polar and bi-polar formats. A uni-polar signal varies between zero volts and a maximum positive voltage or between zero and a maximum negative voltage. A bi-polar signal varies between a maximum negative voltage and a maximum positive voltage generally with zero voltage at the center of the range. Examples of uni-polar signals encountered in aircraft data handling systems are altitude that varies from zero feet to a positive maximum altitude and airspeed that varies from zero knots to a positive maximum airspeed. Examples of bi-polar signals are altitude hold error with respect to a preselected reference altitude where the error signal varies positively and negatively according to whether the aircraft is above or below the reference value and altitude rate that can vary positively and negatively in accordance with whether the aircraft is ascending or descending.
The digital format corresponding to the uni-polar analog signal comprises a set of digital binary digits, or bits, varying from all ZEROS to all ONES, the all ZEROS number corresponding to the zero end of the uni-polar range and the all ONES corresponding to the maximum voltage end of the uni-polar range. The digital bi-polar format comprises a set of bits which range from all ZEROS to all ONES where all ZEROS correspond to one end of the bi-polar analog range and all ONES to the other end of the analog bi-polar range. The zero voltage middle of the range is typically represented by the midvalue digital number consisting of a ONE in the most significant bit position with all of the remaining bits being ZERO.
Integrated circuit digital-to-analog converters are commercially procurable and are commonly utilized in data handling systems as described above. The analog-to-digital conversion function may be performed utilizing a digital-to-analog converter and a comparator to form a conventional successive approximation converter. Such commercially procurable integrated circuit digital-to-analog converters are commonly of the R-2R ladder network variety with a plurality of current steering switches connected to the respective legs of the ladder network. Such digital-to-analog converter integrated circuits commonly provide two current bus outputs, the two poles of each current steering switch being connected to the respective current buses. The binary digits of the digital word to be converted are applied through level shifters to control the states of the respective switches. A binary ONE applied to a switch results in the binary weighted current flowing in the associated leg of the ladder network to be connected to one of the two current buses. A binary ZERO causes the switch to steer the binary weighted current to the other current bus.
Such digital-to-analog chips may be connected to operate in either a uni-polar mode or in a bi-polar mode. In the uni-polar mode a current-to-voltage converter is connected to one of the current buses with the other current bus being grounded. In bi-polar operation two current-to-voltage converters are utilized connected to the respective current buses with the output of one current-to-voltage converter being summed with the input to the other current-to-voltage converter, the output of the other current-to-voltage converter providing the bi-polar device output.
Data handling systems in which both uni-polar and bi-polar data is processed conventionally utilize separate dedicated uni-polar and bi-polar converters at the input and output. For example, such a system may utilize a uni-polar analog-to-digital converter and a bi-polar analog-to-digital converter at the input of the system with address bus control directing the various uni-polar and bi-polar analog signals to the appropriate converter. In a similar manner, the system conventionally utilizes a dedicated uni-polar digital-to-analog converter and a dedicated bi-polar digital-to-analog converter with address control directing the appropriate digital outputs to the appropriate converters.
The dual dedicated converter approach to input/output design is complex and hence expensive, requires excessive printed circuit board area and excessive power and degrades system reliability. An alternative prior art approach utilizes a single bi-polar converter to perform both uni-polar and bi-polar conversions. Bi-polar data is converted as usual, however uni-polar conversions operate over only half the converter's range. If both signal formats must be converted to the same minimum resolution (e.g., n bits) then the converter itself must have twice that resolution (n+1 bits) in order to meet the requirements for the uni-polar case. This method is unduly expensive since cost increases rapidly with converter resolution and accuracy. The prior art also suggests altering the uni-polar signal formats by means of a scaling and offset amplifier so that the full range of a bi-polar converter is utilized. This approach is subject to the disadvantages of drift, scale factor and offset errors.